Laser writer

ABSTRACT

A system and method are provided for microlithographic writing on photosensitive substrates, and especially high precision printing of patterns, such as photomasks, for semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. The method includes the steps of detecting significant temporary writing error conditions and interrupting the writing process as a response to a detection of such an error condition. Thereafter a support table is reversed to the position it had when the writing was interrupted, and the writing process is restarted at the same position where the writing was interrupted when the error condition ceases to exist.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/SE00/00136 which has an Internationalfiling date of Jan. 21, 2000, which designated the United States ofAmerica and was published in English.

FIELD OF THE INVENTION

The present invention relates to a system and a method formicrolithographic writing on photosensitive substrates, and speciallyprinting of patterns with extremely high precision, such as photomasksfor semiconductor device patterns, display panels, integrated opticaldevices and electronic interconnect structures. The terms writing andprinting should be understood in a broad sense, meaning exposure ofphotoresist and photographic emulsion, but also the action of light onother light sensitive media such as dry-process paper, by ablation orchemical processes activated by light or heat. Light is not limited tomean visible light, but a wide range of wavelengths from infrared toextreme UV.

BACKGROUND OF THE INVENTION

A system and method for microlithographic writing of a substrate ispreviously known from e.g. EP 0 467 076 by the same applicant. Ingeneral a system for microlithographic writing, as is shown in FIG. 1,comprises a light source 1, such as a laser, a first lens 2 to contractthe light beams, a modulator 3 to produce the desired pattern to bewritten, the modulator being controlled according to input data, areflecting mirror 4 to direct the beams towards the substrate 6, and alens 5 to contract the beams before the reach the substrate. The mirror4 can be used for a scanning operation to scan the beam over scan linesat the substrate. Several functionally equivalent scanners such asacusto-optic deflectors etc. could also be used. Further, the substrateis preferably arranged on a object table. Two-dimensional relativemotion between the lens 5 and the table (stage) is provided andcontrolled by servo systems. For example the object table could bemovable in at least two orthogonal directions, by means of twoelectrical servomotors. This motion is an inertial mechanical motion,and it is relatively difficult to change the velocity motion quickly.Further, this is irrespective of if it is the object table or the lensthat is moving, since both possess a substantial mass.

However, a problem with such known writing systems are that the emittedoutput power from the laser is not constant, but suffers fromsignificant variations due to so called drop-outs. Such drop-outs aresignificant drops in the light output, typically at least a 25% powerdrop, with a typical duration of about 500 μs, and they occurs onceevery 10 h -1000 h, depending on the type of laser, the age of the laseretc. The problem is especially important for continuous gas lasers.

A short drop-out for about 500 μs can typically result in a loss ofabout 20 scan-lines in the pattern, and such an unwanted hole in thepattern normally ruins the whole mask. For very large patterns, such aslarge-area photomasks, which normally take about 10 hours to write, theproblem is more likely to occur, at the same time as the value of anerror free mask is very high.

Several other problems of the same kind could arise, such as electronicerrors, vibration errors and the like. These problems also generatetemporary interruptions in the continuous writing process.

Other types of pattern generators encounter similar problems, and theinvention is not limited to pattern generators using microlithography.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a systemin which the above mentioned temporary abnormal interruption problems inthe prior art are solved.

This object is achieved with a system according to the appended claims.

By abnormal writing error conditions are in this application meant errorconditions that can be detected and are abnormal in the sense that theycould not in practice be foreseen. Hence, problems that occur at regulartime intervals are not considered abnormal, and neither is for example“data starve”, i.e. when the supply of data is too slow relative to thewriting speed, in a system where this condition can be foreseen fromdata volume and writing mechanics and handled in an ordinary fashion.

SHORT DESCRIPTION OF THE DRAWINGS

For exemplifying purposes, the invention will be described in closerdetail in the following with reference to embodiments thereofillustrated in the attached drawings, wherein:

FIG. 1 is a schematic view of a system according to prior art;

FIG. 2 is a schematic view of a system according to a first embodimentof the invention;

FIG. 3 is a schematic view of a second embodiment of the invention;

FIG. 4 is a schematic block diagram showing the operation of the errorrecovery unit according to the invention;

FIG. 5 is a schematic view of a mask written during a drop-out with asystem according to prior art;

FIG. 6 is a schematic view of a mask written during a drop-out with asystem according to the invention; and

FIG. 7 is a curve representing the output power of a laser versus time.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 2, the system according to a first embodiment of theinvention comprises a light source 1, which is a continuous laser, acomputer-controlled light modulator 3 and a lens 5 to contract the lightbeam from the light source before it reaches the photosensitivesubstrate 6. Further it comprises a second contracting lens 2 ahead ofthe modulator. The laser is e.g. a ion laser, such as a 413 nm, 100 mWkrypton ion laser. The modulator controls the beam according to inputdata 20.

The system according to the invention is a so called “write on the fly”system, where the substrate is placed on a substrate table, and thesubstrate table performs a continuous motion in at least one directionduring the writing process, and the laser beam is at the same timescanned in another direction. The object table has a relatively largemass, and the motion of the same is therefore an inertial mechanicalmotion, which is difficult to control quickly. However, it is alsopossible to move the lens 5 relative to the stage during the writingprocess. This motion will however also be an inertial mechanical motion.The mirror 4 can preferably be used for the scanning operation to scanthe beam over scan lines at the substrate. However, several functionallyequivalent scanners such as acusto-optic deflectors etc. could also beused.

Further the system comprises an error recovery unit 12, and at least oneerror detector 10 (see FIG. 3). The error detector detects significanttemporary errors that arise during the writing process, such as laserdropouts, electronic errors, errors due to vibrations, errors due todata overload etc. In case of a detected error, the detector provides anerror signal 21 to the recovery unit 12.

A position detecting means, such as a counter 13 (see FIG. 3), is alsoprovided, and keeps track of the current position of the beam on thesubstrate. This position detecting means provides a real time positionsignal 22 to the error recovery unit 12.

Furthermore, the error recovery unit 12 is connected to the positioncontrolling means of a stage or the lens 5. Hereby, the recovery unit isable to control the position where the beam hits the substrate, by meansof a position control signal 23. The recovery unit is also connected toa data switch 14, to turn on/off the data stream 20 being supplied tothe modulator, by means of a data control signal 24.

In a more detailed, second embodiment, shown in FIG. 3, tone detectordetects the output power from the laser. Such a detector could betransparent, and be positioned in the beam path. Alternatively it ispositioned away from the beam path, whereby a semi-transparent mirrorcould be used to direct the beam towards the detector. The detector; ispreferably positioned close to the laser.

The detector signal is preferably first forwarded to a filter 11 toavoid noise in the detector signal, and to thereby make the detectormore sensitive for significant variations in the output power. Inaddition, or alternatively, the unit 11 could comprise a comparatorcomparing the detector signal with a threshold value.

The position signal 22 is in this embodiment provided by an incrementalcounter 13, counting the number of bytes that has been forwarded to themodulator since a predetermined starting point. From the counter valuethe current position of the beam spot on the substrate is deducible.

In FIG. 4 a block diagram is shown, presenting the operation of an errorrecovery unit 12 according to the invention. In a first step S1 thesignal from the error detector is evaluated to determine if there exista significant temporary error, such as a significantly decreased laserpower. If such an error does not exist, the data stream will be allowedto continue, by keeping the data switch 14 in the ON condition. On theother hand, if an error does exist, the position data is stored in amemory, in step S2. Thereafter, in step S3, a data control signal 24 issent to the data switch 14 in order to change to the OFF condition.Then, it is determined, in step S4, if the error remains. The operationprocess will not continue to the next step as long as the error remains.When the error signal; from the detector communicates to the recoveryunit that the error condition has ceased to exist, e.g. when the laserpower is regained, the process continues to step S5. Here, the stage isinitiated to a restart of the writing process. This is made by providinga position control signal 23 to the position controlling means for thestage. Hereby, the stage is retracted to the position where the writingwas interrupted. Preferably, a restart of the data supply is alsoinitiated, to make the data delivery unit ready to supply data from thepoint where the error occurred, in step S6. However, this step may notbe necessary if the data delivery unit is directly notified of theinterruption. This could be achieved by forwarding: the data controlsignal 24 to the data delivery unit as well, or if the data switch andthe data delivery unit are integrated. In step S7 it is determined ifthe stage is fully retracted, and ready for a restart. The operationprocess will not continue to the next step as long as the stage is notready. When the stage is fully retracted the writing process isrestarted, in step S8. This is accomplished by forwarding a data controlsignal to put the data switch in the ON condition. As a consequence ofthe stage being retracted, the writing will continue at the same spot asthe error interruption occurred. Hence, there will be no holes or otherdamages due to the error interruption.

In FIG. 5, a schematic example is shown of a substrate where antemporary error interruption has occurred. The writing is made in scanlines, written from the left to the right in the figure, and each lineare written beneath the previous one, due to the continuous upwardmotion of the stage. When a whole stripe is written, the stage is moveddown again, and at the same time slightly to the left, where after thenext stripe is written. During the writing of the second strip the errorhas occurred. The error has caused several scan lines not to be written,and the substrate is severely damaged practically useless. When thewriter system according to the invention is used, the writing processwill be interrupted as soon as the error occurs. The stage is thenstopped and retraced, and thereafter the writing process will continuefrom where the interruption begun, as is indicated by the dashed scanlines. Hence, there will be no errors on the substrate.

In FIG. 7 an example of the generation of an error signal is shownschematically. The detector, in this case a laser power detector,provides a signal indicating the value of output power. The power signalis compared with an threshold value, Th, and during the initial smallpower variations no error signal will be given. However, when, aftersome time, the power drops significantly, an error signal is given tostop the writing process. The error signal will continue until the laserpower is regained, and again is higher than the predetermined thresholdvalue.

However, the detector could instead comprise a deriving filter and,hence, react on the derivative or differential quotient for the signalbeing higher than a predetermined threshold value. Hereby, the detectorwould be very sensitive, and react very fast. However, it might occursome false error detections due to noise etc., but since the systemaccording to the invention takes care of the situation this is just aminor problem. The false error stops will cause no visible signs ormarks on the substrate, and the time needed, and hence the cost, for anextra stop is negligible.

In an alternative embodiment, in the case of a detected abnormal errorcondition, the position for the stage at the time when the erroroccurred is stored. However, the writing process is not interrupted, butis carried through, whereby some parts of the pattern will be missing onthe substrate, such as is shown in FIG. 5. Thereafter, when the writingprocess is completed, a position control signal 23 to the positioncontrolling means for the stage is provided and the stage is retractedto the position is retracted to a position before where the writing wasinterrupted. Thereafter, a restart of the data supply is initiated, tomake the data delivery unit ready to supply data from the point wherethe error occurred, and the writing process is restarted. The writingprocess is then interrupted where the abnormal error condition ceased toexist,land where the previous writing of the substrate were in order.Hence, the holes or other damages due to the error have been repaired.In this alternative embodiment, the data stream could be allowed tocontinue during the time period when the error condition exist, butpreferably it is interrupted, and is not allowed to continue as long asthe error remains. In this embodiment it is further of great importanceto store not only data to be able to identify the position where thewriting process were interrupted, i.e. when the error occurs, but alsothe position where the writing process restarted, i.e. where the errorceased to exist, or, in the case where the data stream was interrupted,where the data flow was continued to be supplied.

It should be noted, that it is also possible to perform the slowrelative motion by moving the lens 5 instead of the stage 6, even thoughthe stage is moved in the examples given above. It should also bepossible to have other detectors, or even several different detectors,connected to the error recovery unit, detecting different abnormalerrors likely to occur during the writing process. Such abnormal errorscould be

transients in the laser;

bit errors in the data supply, which could be detected by for examplechecksum controls;

too large mechanical vibrations, such as mechanical vibrations exceedinga predetermined threshold value.

The position detecting means could be a counter for counting the numberof scan lines being written, the number of clock cycles the writingoperation has lasted etc. However, other position detecting means couldbe used. Several other variations of the above-mentioned embodiments arealso possible, and obvious for a person skilled in the art.

Further, the invention has above been described with reference to amicrolithography system, where photons (light) is used as the radiantenergy. However, other types of radiant energy may be used as well, suchas charged particles, electrons, ions, EUV (Extreme Ultra Violet), andother radiant energies suitable for substrate exposure. Further, othertypes of pattern generators may be used. For example, the patterngenerator may use an acousto-optic modulator to control the radiantenergy according to input data, as well as a voltage controlled trolledmodulator or an SLM (Spatial Light Modulator). If charged particles areused, the pattern generator may either be of the type using rasterscanning or the type using shaped beams. In the case when an SLM isused, an abnormal error condition could be the case of unwanted absenceof light pulses, or errors in the input pattern data. When such errorsin the input data are detected, which could be accomplished by achecksum test, the immediate response action could be to load zeros tothe modulator, to get a zero exposure, or to stop the light flash frombeing emitted, by not transferring a trigger signal to the light source.

Such obvious modifications must be considered as being part of theinvention, as it is defined by the following claims.

The invention makes the production of large area display panels moreefficient and cost effective by increasing the yield of the productionand decreasing the needed rewriting. At the same time the patternprecision is improved. Further, the invention makes it possible to useeach laser for a longer period of time, before it need to be exchanged.

What is claimed is:
 1. A pattern generator system for writing ofpatterns on a substrate (6) with a layer being sensitive for a radiantenergy, the system comprising a source (1) for emitting such radiantenergy, a computer-controlled modulator (3), a lens for creating apattern by the radiant energy on the substrate, and a substrate supporttable to support the substrate, whereby the writing operation comprisesa relative motion between the lens (5) and the support table (6) duringthe writing operation, which motion involves an inertial mechanicalmotion, and further comprising a detector (10) for detecting abnormaltemporary writing error conditions, and an error recovery system (12)for enabling a restart of the writing process at a position where thewriting was interrupted when the error condition has ceased to exist. 2.A system according to claim 1, wherein the error recovery system furthercomprises an interrupter (12) responsive to said detector for stoppingthe writing process in case of said error writing condition, forenabling a restart of the writing process at the position where thewriting was interrupted when the error condition has ceased to exist. 3.A system according to claim 2, wherein the interrupter comprises acounter (13) counting the number of scan-lines being written.
 4. Asystem according to claim 2, wherein the interrupter comprises a counter(13) counting the number of clock cycles during the writing operation.5. A system according to claim 2, wherein the recovery system comprisesstorage means (13) for storing information from which is deducible thewriting position where the abnormal error condition occurred.
 6. Asystem according to claim 1, further comprising means for retracting thesupport table and restarting the exposure at the position where thewriting was interrupted directly after the error condition has ceased toexist.
 7. A system according to claim 1, further comprising means forretracting the support table and restarting the exposure at the positionwhere the writing was interrupted when the writing process is completed.8. A system according to claim 7, wherein the recovery system comprisesstorage means (13) for storing information from which is deducible thewriting position where the abnormal error condition ceased to exist. 9.A system according to claim 1, wherein the detector is a detector fordetecting significant changes in the emitted output power from thelaser.
 10. A system according to claim 1, wherein the detector comprisesa threshold value comparator, whereby only changes exceeding a certainthreshold value give rise to a signal affecting the interrupting means.11. A system according to claim 1, wherein the detector is a detectorfor detecting electronic errors.
 12. A system according to claim 1,wherein the detector is a detector for detecting bit errors in thesupplied data, and preferably a checksum detector.
 13. A method forwriting of patterns with a pattern generator system on a substrate (6)with a layer being sensitive to radiant energy, the system comprising asource (1) for emitting radiant energy, a computer-controlled modulator(3), a lens to create a pattern by the radiant energy on the substrate,and a substrate support table, which is movable in at least onedirection and performs an inertial mechanical motion during the laserscanning operation, c h a r a c t e r i s e d by the further steps ofdetecting significant abnormal temporary writing error conditions, andsubsequently restarting the writing process at the position where thewriting was interrupted when the error condition has ceased to exist.14. A method according to claim 13, comprising the further step ofinterrupting the writing process in case of an abnormal writing errorcondition, for enabling a restart of the writing process at the positionwhere the writing was interrupted when the error condition has ceased toexist.
 15. A method according to claim 14, wherein the interrupting stepcomprises the further step of storing information from which isdeducible the writing position where the abnormal error conditionoccurred.
 16. A method according to claim 13, further comprising thestep of retracting the support table and restarting the exposure at theposition where the writing was interrupted directly after the errorcondition has ceased to exist.
 17. A method according to claim 13,further comprising the step of or retracting the support table andrestarting the exposure at the position where the writing wasinterrupted when the writing process is completed.
 18. A methodaccording to claim 17, further comprising the step of storinginformation from which is deducible the writing position where theabnormal error condition ceased to exist.
 19. A method according toclaim 13, wherein significant changes in the emitted output power fromthe laser are detected in the step of detection.
 20. A method accordingto claim 13, wherein electronic errors are detected in the step ofdetection.
 21. A method according to claim 13, bit errors in thesupplied data are detected in the step of detection.